1. Field of the Invention
The present invention concerns radar antennas for the transmission or reception of microwave energy and, notably, so-called multiple-beam antennas capable of emitting or receiving energy in space according to different radiation patterns.
Multiple-beam antennas such as this are used, for example, to simultaneously establish mutually orthogonal radiation patterns such as a pattern called a "sum pattern" having a very narrow main lobe in one direction of aim and a pattern called a "difference pattern" with a very substantially weakening in this direction of aim and with two narrow major lobes on either side of this direction
2. Description of the Prior Art
One approach commonly used for making such antennas consists in providing for an array of radiating elements supplied by an energy distributor known as a Blass matrix. This distributor has several main energy leading-in lines and several secondary lines which intersect the former and lead to the array of radiating lines. Couplers are placed at each intersection so that a fraction of the incident energy on a main line is directed towards a secondary line in a well-determined direction, towards a radiating element placed at one end of this secondary line. The other end of this secondary line is provided with an absorbent load. Between two intersections of a secondary line with the different main lines, i.e. between the directional couplers corresponding to these intersections, phase shifters which may be line sections are inserted. The coupling coefficients of the different couplers and the phase-shifting values of the different constant or variable phase-shifters are computed so as to obtain the desired radiation patterns which differ according to whether the energy arrives by or is picked up by either of the main lines. Herein, we refer only to the operation of the antennas in transmission, but it is clear that they can work in reception too.
The Blass matrix is very frequently used, but it is costly and complicated to make, notably because of the directional couplers that have to be provided at each intersection.
A known way of overcoming this drawback is to eliminate the physical connection with the secondary lines. We then speak of an optical Blass matrix. In these optical Blass matrices, the secondary lines are replaced by direct radiation in space from the main lines which are the only ones remaining. This radiation caused by obstacles on the main lines is propagated directionally on only one side of the main lines with a degree of coupling and a phase shift that varies along each of the main lines, the variation curves of the coupling and of the phase-shifting being different for the different main lines so as to set up different radiation patterns, for example a sum pattern and a difference pattern at one and the same frequency or, again, two sum patterns corresponding to two different frequencies.
There is a known Blass matrix type microwave energy distributor comprising:
an elongated open guide provided with parallel side walls forming an elongated hollow structure open on one side essentially on its entire length,
an absorbent load placed all along the guide between the side walls,
at least two elongated, thin conductive strips placed between the load and the open side of the guide, parallel to the length of the guide, these strips being used as the main energy propagation lines and
asymmetrical obstacles distributed in the cavity along the guide and formed by tongues cut out on the sides of the conductive strips and folded crosswise to these strips, said obstacles generating a radiation propagated towards the opening of the guide.
This known microwave energy distributor of the optical Blass type has the drawback of having radiating obstacles formed by thin tongues that have a low passband and cannot be used to form extensive couplings. This entails a penalty when making illuminations with a small number of sources. It also possesses the drawback of having obstacles that have to be manufactured with the main lines, making it necessary to have precise folds with respect to the perpendicular of the plane of these main lines.